Error diffusion is representative pseudo halftone processing for binary representation of multivalue image data (“An Adaptive Algorithm for Spatial Gray Scale” in society for Information Display 1975 Symposium Digest of Technical Papers, 1975, 36).
This scheme is converting input multivalue image such that the density of entire multivalue image and the density of entire binarized image are substantially the same, i.e., the density is preserved, as briefly described below.
Assuming that a pixel of interest P has a density V, and unbinarized peripheral pixels P0, P1, P2 and P3 have densities V0, V1, V2 and V3, and further, a threshold value for binarization is T, a binarization error E in the pixel of interest P is distributed to the positions of the peripheral pixels P0 to P3 by using weight coefficients W0, W1, W2 and W3. Assuming that the maximum density is Vmax, the minimum density, Vmin and binarized data of the pixel of interest is B, the above distribution is expressed as follows.If V≧T holds, B=1, Error E=V−VmaxIf V<T holds, B=0, Error E=V−Vmin   (1)Then, the values V0 to V3 upon distribution to unbinarized pixels P0 to P3 are,V0=V0+E×W0V1=V1+E×W1V2=V2+E×W2V3=V3+E×W3
Note that to equalize the density of the entire image binarized from the input image, generally, W0+W1+W2+W3=1 holds. For example, W0= 7/16, W1= 1/16, W2= 5/16 and W3= 3/16 hold.
By using this scheme, even if a printer which merely print-outputs an input multivalue image by printing/not printing dots is used, a print output perceivable to human eye as an image having gradation (tonality image or halftone image) can be realized.
Regarding a color image printing, a color printer generally performs printing by using ink or toner (printing material) of yellow (Y), magenta (M), cyan (C) and black (Bk) color components. Accordingly, binarization is performed based on the above error diffusion scheme by color component and heads of respective printing components are driven based on the binarized results.
However, in a color image, a color is represented by a balance among 4 (3 if Bk component printing material is not used) color components. Accordingly, binarization independently on respective color components equals binarization without consideration of other color components. For example, if two or more colors often overlap with each other in a comparatively low-density region, satisfactory image quality cannot be attained.
As an example of improvement in this problem, Japanese Laid-open Patent Publication Nos. 8-279920 and 11-10918 are known. These publications disclose pseudo halftone processing for obtaining excellent visual characteristics even in a case where two or more colors overlap with each other by using error diffusion in combination of two or more colors.
However, in the method disclosed in the above publications, upon determination of output corresponding to an input pixel, an output value is determined by a comparator or the like. Accordingly, for example, if changing of output gray scale level is realized by hardware, the circuit is complicated, and if the above function is realized by hardware, the program size is large or more memory capacity is required.
Further, if output gray scale levels of two colors are different, e.g., cyan output is made in quaternary representation but magenta output is made in binary representation, or if the number of output colors is to three or more colors, the circuit or program must be greatly changed.